Correlations have been studied between evaporative H/He emission and fissionlike products for the reactions 333 MeV Ar+ ' Sn, ""Sm, and ' Au. Fission products were detected at 60 and 320 in coincidence with H and He at 120 in plane and 27 and 60' out of plane. The coincidence energy spectra of H and He are not consistent with evaporation from the moving fragments. An analysis has been made with the assumption of complete energy equilibration in the particle emission (i.e. , evaporation) prior to scission. The rather large data set is essentially consistent with this assumption. From the spectral shapes one can deduce level density parameters and barriers to evaporation. Values inferred for the Fermi gas level density parameters are similar to those inferred from experiments at much lower energies. Values deduced for the emission barriers are much smaller than those from systematics of fusion barriers. The observed cross sections for fission and 'H and He evaporation are not easily reconciled with statistical model calculations. In particular, the proton evaporation seems to compete more favorably with fission and He evaporation than implied by phase space considerations alone. NUCLEAR REACTIONS " Sn, ' Sm, ' Au( Ar; H/He, fission), E =333 MeV, measured energy and angular correlations of evaporative H/He in coincidence with fission. Values deduced for emission barriers, temperatures and J",for the emitter. Comparisons made to evaporation calculations.
Experimental results on the limits observed for complete fusion between heavy nucl. ei are discussed. It is shown that there is no critical angul. ar momentum independent of the bombarding energy and of the entrance channel. Excitation energy and total. angular momentum of the compound system do not appear as good parameters for determining the complete fusion cross section. A'll the available results can be presented in terms of a critical distance of approach R~. Such a distance is obtained when the kinetic energy of incoming nucleons equals the interaction potential and i.s related to the nuclear-matter density of both nuclei.For very heavy partners, a fusion barrier occurs, in addition to the interaction barrier. NUCLEAR REACTIONS Heavy-ion complete fusion; calculated critical distance of approach, 8 =(1.0+0.07)(A(~3+&2~~3). COMPLETE FUSION AND CRITICAL ANGULAR MOMENTUMWhen nuclear reactions are induced by heavy ions, an important question is which part of the total reaction cross section goes into complete fusion and then leads to a compound nucleus. It has important consequences for the yield for the production of new isotopes and in itself it might give information on the dynamical properties of nuclear matter.A number of experimental results' ' have been obtained during the last decade, mainly on the ratio (o«/8") of the complete-fusion cross section as compared to the total reaction cross section. The first conclusion was that the completefusion cross section did not always represent the largest fraction of the total cross section and a reasonable explanation was given by considering the large angular momenta brought into the compound system by heavy projectiles of large momenta. The concept of a critical value of angular momentum was stated by several authors. " However, more recent data have shown that such a critical limit is certainly not characterized by only the properties of the compound system. In particular, experiments by Zebelman et al. '0' " have very clearly demonstrated that the entrance channel had to be considered since, for the same compound nucleus at the same excitation energies (",00Yb) four different critical LK were deduced from o cF measurements, when four different projectiles ("B,~C, "0, ' Ne) were used. Similar results have been obtained at Orsay' with a completely different experimental method of determination of l;, S. For the formation of ' VTe at the same excitation of 107 MeV energy, l";, was found equal to 70 when the projectile was 40Ar, and only 50 with '4¹ A second set of results has also very clearly shown that for a given system, l";, does not stay at a constant value whatever the bombarding energy and, therefore, the excitation energy of the compound system. If a limiting of lk is found at a given ener gy and if such a critical lh is a constant, then by increasing the energy, oc"/os should decrease at the same rate as (L";,/L ")'.Although the first experiments' could give the feeling that this was the case, new data from Natowitz, Chuliek, and Namboodiri' with C, N, ...
Abstract. Events with 2, 3 and 4 heavy fragments (A > 20) detected in the reactions ~~176 + l~176 at 18.7, 23.7 A" MeV and ~2~ + ~Z~ at 18.4 A' MeV were analyzed by means of an improved version of the kinematic coincidence method. The phase-space distributions prove that 3-(and possibly 4-) body events predominantly originate from a two-step mechanism and are compatible with the hypothesis of a binary deep-inelastic interaction followed by the further fissionlike decay of one (or both) of the primary fragments. The characteristics of the fission step -mass asymmetry, relative velocity, in-plane and out-of-plane angles -have been reconstructed for the 3-body events and indications are found that nonequilibrium effects at the end of the deep-inelastic phase may influence the fissionlike decay.
We have analyzed a large set of mean energies and angular anisotropies for evaporative 4He emission to obtain barriers to evaporation, B. These exit channel barriers are often substantially smaller than the corresponding empirical s-wave fusion barriers E o. The differences (Eo-B) are interpreted as indicators of the extent of distortion of the emitters. These distortions have in turn been characterized by the deformation parameter for a spheroid %0. For Z=80 the dependence of B or %0 on spin is somewhat suggestive of the superdeformed shapes predicted by the liquid drop model. For Z > 70 significant distortions are indicated for emitters of both large and small spin.How can one get information about the sizes and/or shapes of composite nuclei formed in collisions between heavy ions? This question is of great importance for the understanding of nuclear reactions involving high energies and spins [1]. In several recent papers it has been shown that evaporation-like 4He emission often occurs at a very early stage in the reaction [-2-4], suggesting that these emissions may offer a promising probe of the early evolution of the reaction mechanisms. To explore this possibility we investigate the systematics of some of the experimental properties of these evaporation-like processes, the mean energies ({,)) and anisotropy parameters /32. The data are analyzed with a simple procedure to obtain values for the effective barrier to evaporation, B. These exit channel barriers are then compared to the s-wave fusion barriers Eo for *He [5,6]. The sizable differences (Eo-B) between the two lead us to conclude that in many cases the excited emitter nucleus must be quite deformed (or swollen) compared to its unexcited counterpart in the fusion reaction. As an example we estimate a corresponding quadrupole deformation parameter for evaporation from the tips of a prolate spheroidal emitter as compared to the fusion of spheres. These deformation parameters cannot be taken literally but only as * Permanent address: Institut de Physique Nucleaire, B.P. No. 1, F-91406 Orsay, France semiquantitative indicators of the extent of distortion. They are surprisingly large, and their dependence on Z, energy, and spin shows quite interesting trends. In the first five columns of Table 1 we give the experimental data employed [2,3,[7][8][9][10][11][12][13]. In the others we give quantities derived from this analysis. The basic experimental quantities are the anisotropy parameters /32 and the mean energies {(e)) for 4He, observed in the singles mode at angles greater than 90 ~ c.m. The anisotropy parameters f12 were obtained by fitting the angular distributions to equations from the statistical model [3,10]. If this distribution is isotropic then /32 is zero; increasing forward-backward peaking is characterized by increasing /32-We will employ equations derived for evaporation from a spherical emitting nucleus at a given excitation energy E* and spin J. Therefore we must estimate the effective Z, A and E* of the emitters, and for high Z we must con...
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